Carbon and nitrogen mineralization from added organic matter in saline and alkali soils

Abstract

The mineralization of N in salt-affected soils is a subject of much controversy because there are reports of salt induced non-biological ammonification. In laboratory experiments we studied the decomposition of green manure Sesbania in saline and alkali (sodic) soils. Carbon mineralization was reduced from 38.6 to 16.8% when electrical conductivity (ECe) in saline soils was 97 dS m⁻¹. In alkali soils C mineralization was ∼38.0% irrespective of pH variation from 8.1 to 10.0. Ammoniacal-N accumulated up to ECe 70 dS m⁻¹ beyond which it declined; nitrite+nitrate-N declined at ECe 16 dS m⁻¹ and was not detectable at ECe≥26 dS m⁻¹. Net N mineralized decreased from 351.2 mg kg⁻¹ in the non-saline control soil (ECe 1.1 dS m⁻¹) to 277 mg kg⁻¹ at ECe 97 dS m⁻¹ salinity and from 399.2 in control (pH 8.1) to 317 mg kg⁻¹ at pH 10.0 in alkali soils. There was no NH₃ volatilization in the saline soils and it was negligible in the alkali soils; most of the N lost was presumably due to denitrification. Protease, amidase (asparaginase, glutaminase) and deaminase (histidase) activities in soil were low but were stimulated by organic matter addition. Glutaminase activity was appreciable at high pH and deaminase at high salinity. The depression of ammonification at ECe 97 dS m⁻¹ and the overall decrease in net N mineralization with increasing pH or salinity, and the significant enzymatic hydrolysis of organic N at high pH or salinity were all evidence of the biological nature of N mineralization in salt-affected soils. The adaptation of nitrifiers to salt stress at later stages of incubation, the nature of N losses and the reduction in losses with increase in pH or salinity also indirectly showed that N mineralization was biological and not chemical.

Introduction

Salt-affected soils are an important ecological entity in the landscape of semi-arid and arid regions of the world. Excessive amounts of salts have adverse effects on soil physical and chemical properties and microbiological processes. These include effects on carbon and nitrogen mineralization and soil enzyme activities which are crucial for the decomposition of organic matter and release of nutrients required to sustain productivity. Ammonification is stimulated at low salt concentrations and inhibited at higher amounts but nitrification is very sensitive to salinity (Westerman and Tucker, 1974; McCormick and Wolf, 1980; McClung and Frankenberger, 1987). Laura (1974)observed that C mineralization from added green manure was inhibited as salinity increased from 0.1 to 5.1% NaCl but ammonification continued at the same rate. He hypothesized (Laura, 1975) that this mineralized N was not biological in origin but physico-chemical, caused by the protolytic action of water on organic nitrogenous constituents of soil organic matter. He found further evidence in alkali (sodic) soils (Laura, 1976) in support of this hypothesis.

While reviewing Laura's and other arguments on the apparent priming effect of salts on N mineralization, Schnitzer and Khan (1980)concluded that this subject is in need of increased investigation. Unlike a true priming effect the influence of salts on N mineralization could be more dramatic and of greater practical significance, because increased mining of soil N and increased nitrate pollution are highly undesirable.

Mineralization of nitrogen at high salinity and alkalinity has so far been ascribed to either biological (McCormick and Wolf, 1980) or chemical mechanisms (Laura, 1975). Even if microbial activity is depressed by salts, biochemical mineralization by soil enzymes (amidases and deaminases) could still proceed provided the activity is not adversely affected at high salinity and alkalinity. This possibility has not been considered previously. We therefore repeated Laura's experiments and studied the (a) effects of salinity and alkalinity on C and N mineralization in soils amended with the green manure Sesbania and (b) activity of soil enzymes involved in biochemical mineralization of N by breakdown of protein, amides and amino acids in salt-affected soils.